A liquid crystal display device is provided with a backlight unit for irradiating a liquid crystal panel (LCD) with light from its rear side and illuminating the liquid crystal panel. The backlight unit includes an illuminating light source, a light guide plate that diffuses light emitted from the light source to irradiate the liquid crystal panel with the light, a prism sheet or diffusion sheet for equalizing the light emitted from the light guide plate and other such components.
In recent years, there is an increasing demand for reduction in thickness and power consumption of a liquid crystal display device. To meet such a demand, light guide plates of various shapes have been proposed (refer to JP 9-304623 A, JP 8-62426 A, JP 10-133027 A, and JP 5-249320 A)
FIG. 17 is a schematic sectional diagram of an area light source device including a light guide plate 100 disclosed in JP 9-304623 A.
The area light source device (backlight unit) shown in FIG. 17 is obtained by embedding a fluorescent lamp 102 in the light guide plate 100, then arranging a reflective sheet 104 on a rear surface of the light guide plate 100 and laminating a transmitted light amount correction sheet 106, a light diffuser 108, and a prism sheet 110 on a light exit surface of the light guide plate 100.
The light guide plate 100 has a substantially rectangular shape and is made of a resin into which fine particles functioning to diffuse illumination light are dispersed. Also, the light guide plate 100 has a flat upper surface, which is used as the light exit surface. Further, grooves 100a each having a U shape in section are formed in the rear surface (surface opposite to the light exit surface) of the light guide plate 100. The fluorescent lamp 102 is fitted into each groove. A light amount correction surface 100b for promoting the emission of illumination light is formed except portions just above the fluorescent lamps 102.
As mentioned above, JP 9-304623 A describes that, the fine particles are mixed into the resin to form the light guide plate 100 and in addition, the illumination light is promoted to exit by the light amount correction surface 100b formed on a part or all of the light exit surface except the portions just above the fluorescent lamps 102, making it possible to reduce the total thickness and unnatural and uneven luminance of the light emitted from the light guide plate 100.
For the purpose of obtaining a backlight of a liquid crystal display device, which enables reduction in size, weight, and thickness of the liquid crystal display device as well as cost and power consumption without reducing the illumination amount of the backlight, JP 8-62426 A discloses a light guide plate having a rectangular illumination surface, a groove rectangular in section which is gouged out at a central portion with respect to the short sides in parallel with the long sides and in which a light source is inserted, and a rear surface formed such that a plate thickness is gradually reduced from the groove toward both lateral surfaces on the long sides.
In addition, for the purpose of obtaining a backlight unit that enables a thin frame and reduction of the total thickness of a liquid crystal display device, and is high in use efficiency and luminance, JP 10-133027 A discloses a light guide member (light guide plate) having a parabolic shape in section along the width direction of a concave portion where the light source is arranged, with the depth direction being a major axis.
Further, JP 5-249320 A discloses a light guide plate where plural plate-like optical waveguide layers are laminated so as to have successively increased refractive indexes on a high reflectance layer arranged in substantially a V shape, and light emitted from each light exit surface illuminates a light diffusion layer for the purpose of maintaining even in-plane brightness of a display panel and attaining high-luminance illumination. Here, a concave portion where the light source is arranged has a triangular shape.
The light guide plates disclosed in the patent documents mentioned above aim at some of slimness, reduction in size and weight, low power consumption, and low cost. In any of the plates, one or more grooves are formed at the central portion, and the bar-like light source is accommodated in the groove. The plates are preferably designed such that the plate thickness is gradually reduced from the groove portion toward the end surfaces, thus attaining the reduction in thickness.